The subject matter disclosed herein relates to refrigeration systems. More specifically, the subject matter disclosed herein relates to determination and monitoring of refrigerant levels in refrigeration systems.
Fruits, vegetables and other perishable items, including meat, poultry and fish, fresh or frozen, are commonly transported in the cargo box of a truck or trailer, or in an intermodal container. Accordingly, it is customary to provide a transportation refrigeration system in operative association with the cargo box for cooling the atmosphere within the cargo box. The transport refrigeration system includes a refrigerant vapor compression system, also referred to as a transport refrigeration unit, and an on-board power unit. The refrigerant vapor compression system typically includes a compressor, a condenser, an expansion device and an evaporator serially connected by refrigerant lines in a closed refrigerant circuit in accord with known refrigerant vapor compression cycles. The power unit includes an engine, typically diesel powered.
The system is charged with a selected volume of refrigerant, which is held in a receiver, or holding tank, sufficient to allow for a desired amount of refrigeration over a wide range of operating temperatures. In a typical system, the receiver includes one or more sight glasses to allow an operator to visually assess the refrigerant charge level present in the receiver. When adding refrigerant in the field (when refrigerant charge in the system cannot be weighed), sight glasses are used to verify a correct amount of refrigerant added to the receiver to result in an acceptable level of refrigerant in the receiver. Sight glasses are also used to periodically determine if the system is over charged or under charged. Accuracy in determining the level of refrigerant charge via the sight glasses can be unreliable, as the method is dependent on the conditions under which the system is operating when the refrigerant level is checked. At certain operating conditions, the system can appear to be undercharged or overcharged based on the sight glass observation, but may in fact be properly charged.